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How many solar panels do I need to load 16 eve Lifepo4 batteries (48v system)?

Messier11

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Hi everyone,

I am a true newbie and I am trying to figure out how many panels I should plan to get for a system based on 16 pcs lifepo4 of 3.2v and 280ah each Alibaba style.

What is the recommended load current for these batteries?

I of course understand that the number of panels I need depends on what type of panels I get. But how should I calculate on it? Let’s say I get 150w 12v panels? I am sure the information is right infront of me, but I can’t figure out how to do the math. Other batteries I have looked at have stated for example that the recommended load current is 37A, which I gather means that I should for example have three 150w panels to get optimal charging (3 x 150w/12v = app. 37a). But I can’t find that number for these batteries.

I am looking at batteries like these (perhaps through Amy):
 
Your Solar array size is more dependent on energy usage than on battery size. If you only use 100Wh a day, then a small 100W panel would be more than enough. If you use 2000Wh per day, the array size will need to be much larger.

My advice to all beginners is to do an energy survey/audit before you do anything else. This will clarify the picture of what the energy usage is and help you size the system. This is the tool I use for an energy audit:

 
I need 40kwh to go with my 13kw PV array to be able to use 100% of the PV power the panels generate. What I mean by this is that the battery has to be big enough on prime days (spring/early-summer) to take in all the PV but spread out consumption thru the night till the next day.

There are many factors but your battery (280ah @ 52v) is aprox 14.5kwh and a simple extrapolation would be 14.5kwh/40kwh = 36% of my battery capacity. Applying 36% to 13000kw PV = 4650w PV array. 4650w / 52v = 89a.

This suggests that if you did a 4650w PV array you could maximize your solar power input and it would require 90a max charging - which should be in range. Of course, check your specs and this is just a general estimate. You're consumption and available sun may be wildly different than mine.
 
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Generally speaking, LiFePo4 can handle .5c and 1C charging. Check the spec for the cells you are buying.
If you are building a 48V battery:

.5C of 280aH cells is 140A
1C of 280aH cells is 280A

Then check the BMS(s?) you are using. It may have a lower charging limit.
 
Your Solar array size is more dependent on energy usage than on battery size. If you only use 100Wh a day, then a small 100W panel would be more than enough. If you use 2000Wh per day, the array size will need to be much larger.

My advice to all beginners is to do an energy survey/audit before you do anything else. This will clarify the picture of what the energy usage is and help you size the system. This is the tool I use for an energy audit:

-
I need 40kwh to go with my 13kw PV array to be able to use 100% of the PV power the panels generate. What I mean by this is that the battery has to be big enough on prime days (spring/early-summer) to take in all the PV but spread out consumption thru the night till the next day.

There are many factors but your battery (280ah @ 52v) is aprox 14.5kwh and a simple extrapolation would be 14.5kwh/40kwh = 36% of my battery capacity. Applying 36% to 13000kw PV = 4650w PV array. 4650w / 52v = 89a.

This suggests that if you did a 4650w PV array you could maximize your solar power input and it would require 90a max charging - which should be in range. Of course, check your specs.

Many thanks!

In this case, it’s for an off grid cabin that we only stay at for about 6 weeks during the summer and a couple of weeks during the winter. It’s hard to map how much power we will use if we get 230v (currently it’s just 12v for loading cell phones and a few lights). Sometimes we are 5-6 people there, sometimes 20+.

But would it be possible to load the system with like eight 150w 12v panels? Or is a minimum number of cells required? I mean, we have a gasoline generator, if needed I can can fire that up and load the batteries if we get cloudy weather or the usage has been unusually high.
 
Many thanks!
This is solid forum / many contributors, you'll get solid info here beyond my comments - but thanks.

But would it be possible to load the system with like eight 150w 12v panels? Or is a minimum number of cells required? I mean, we have a gasoline generator, if needed I can can fire that up and load the batteries if we get cloudy weather or the usage has been unusually high.
8 * 150w panels = 1200w. 1200w / 48v (battery) = 25a. I can pretty much quarantee that 16s1p of 280ah cells can accept that much charge :)

One thing to keep in mind, the panels must be serialized to produce >60v - e.g. the solar voltage has to be higher than the max battery voltatge. You can't charge a 48v battery from 12v PV input.

So you'll want to do at least 2 strings of 4 panels in series (2s4p). If you post the specs we can answer weather that will work. You might have to do all 8 in series to get a high enough voltage.
 
If you are using an MPPT controller for the system, the only 'minimum' is that the panels must be high enough voltage to start the controller..... but even 12V panels will create a high enough voltage for a 12V system.

Even if you have to do a lot of guesses.... I highly recommend you go through an energy survey.
 
You say "16 pieces" - but could you clarify if you intend to do 16s1p - e.g. 48v battery or 8s2p 24v battery or 4s4p = 12v battery. This will help get you proper info.
 
You say "16 pieces" - but could you clarify if you intend to do 16s1p - e.g. 48v battery or 8s2p 24v battery or 4s4p = 12v battery. This will help get you proper info.

Of course, it’s 16 batteries with 3.2v each, so 48v in total (if I understand your question correctly). I am both a newbie and English is my second language. :)
 
With a 48v system and 15kwh of battery, I think you are wasting your time with small panels. Based off the controller you mentioned, I would go with x10 300w panels minimum (2x 5S1P), and if possible, run the two arrays pointing in different directions to extend your solar day as much as possible. Ultimately as mentioned above, you only need as many panels as you have usage, but more is almost always better to get you through cloudy days or winter.

My system has x9 250w panels (3 series, 3 parallel) and will generate 8-10kwh on a good day. 1.5kwh of that goes into the inverter itself.
 
With a 48v system and 15kwh of battery, I think you are wasting your time with small panels. Based off the controller you mentioned, I would go with x10 300w panels minimum (2x 5S1P), and if possible, run the two arrays pointing in different directions to extend your solar day as much as possible. Ultimately as mentioned above, you only need as many panels as you have usage, but more is almost always better to get you through cloudy days or winter.

My system has x9 250w panels (3 series, 3 parallel) and will generate 8-10kwh on a good day. 1.5kwh of that goes into the inverter itself.

Thanks!

I am very much a newbie, does the specs of the gear I listed allow for two arrays pointing in different directions? It won’t be a problem if half the panels are shadowed while the other half is in the sun?
 
In their documentation, Victron recommends ~72 cells in series for 12V systems, ~144 cells in series for 24V systems. They do not make a reccomendations for 48V systems as far as I've seen but it seems reasonable to extrapolate ~288 cells. 288 cells is 4 x '24V' panels or 8 x '12V'

These are are their recommendations, they also cite minimums which are half the number of the recommended values.
 
In their documentation, Victron recommends ~72 cells in series for 12V systems, ~144 cells in series for 24V systems. They do not make a reccomendations for 48V systems as far as I've seen but it seems reasonable to extrapolate ~288 cells. 288 cells is 4 x '24V' panels or 8 x '12V'

These are are their recommendations, they also cite minimums which are half the number of the recommended values.

So would 6 of these work if I have 4 aimed at South and 2 at west?
332BDD95-3D2F-4993-80C2-02062D9B77B6.png

They are about 200 USD each.
 
I have only barely skimmed this thread and don't the specifics of your situation, and I don't know what charge controller you have, so I'm going to take the "teach a man to fish approach" (I'm not sure how well that will translate to your native language).

An MPPT charge controller, can be thought of like a fancy buck converter. Meaning, it converts a higher voltage input (from the PV array) down to a lower voltage output (the battery bank voltage).

So the first condition that must be met when sizing your array, is that the voltage of the array must be higher than the voltage of the battery bank (+ some overhead) in order for the MPPT controller to operate and supply power.

Next you have to look at the input limits for your charge controller.
The most important limit is the maximum input voltage, your solar array cannot exceed this value, and you have to account for cold weather (the easy and cautious way to do this is multiply the array Voc * 1.25 or so). Next you have to look at the input, the input current should not exceed the maximum current rating of the solar charge controller. Additionally some charge controllers will have a maximum power rating, if so, you also need to make sure the maximum power of your array does not exceed the maximum power rating of the charge controller.

Finally, you need to consider output. Can your charge controller pass through the full current of your PV array. You take the power (Watts) of your PV array and divide by the battery bank voltage. So for instance 2048W / 51.2V = 40A output. Its okay if this value is a little above the current rating of your charge controller so long as you understand it may be a bottleneck during peak conditions. But as long as all other limits are followed it is safe to do so.

This may sound complicated, and sort of is at first but the basics are actually pretty simple. Its mostly just a matter of comparing between two datasheets (your charge controller and your PV panels) and doing some simple math.

I'm sure there are many good explainer articles on this, unfortunately none come to mind right now, maybe someone else can give a link to a good intro. The best I can find so far is this article from Victron, but its more specific to overpaneling
 
So would 6 of these work if I have 4 aimed at South and 2 at west?


They are about 200 USD each.

Specs on those panels say:

Watts (STC) 445 W
Max Power Voltage (VMPP) 41.3 V
Max Power Current (IMPP) 10.71 A
Open Circuit Voltage (VOC) 49.3 V
Short Circuit Current (ISC) 11.53 A

The Victron 250/60 says:

1642351772782.png

If you use the panel you noted above, I would set the array up as 4 panels in series and run two strings (8 panels total). The controller has two inputs. This will give you 1780w per string (3560w total) on a perfect day.

You can use the Victron calculator to run the numbers and see which solution makes the most sense for you.

 
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